The present invention relates to the field of concretes, more particularly to fibre-reinforced concretes. The main subject of the invention is an improved concrete, especially making it possible to manufacture elements of civil engineering structures, intended for constructing buildings and highway structures, and having properties superior to those of elements in the prior art. In particular, the present invention aims to obtain, for structural concretes, mechanical behaviour which is tough and ductile at the same time.
Structural analysis of concretes has shown that their mechanical properties are intimately linked to the presence of structural defects. Several types of defects distinguishable by their size may be observed in these concretes when they are subjected to mechanical loads.
On a smaller scale, the defect called microporosity is observed in concrete. This consists of pores, called capillaries, emanating from the intergranular spaces initially present in the fresh paste. Their size varies between 50 nm and a few xcexcm.
On the next scale up, microcracking defects are observed. These are microcracks having openings ranging from 1 xcexcm to a few hundreds of Am. They are non-coalescent, that is to say that they do not form a continuous path through the structure. They are mainly due to the heterogeneous character of concrete, the aggregate having mechanical and physical properties different from those of the binder/cement. These microcracks appear during mechanical loading. This type of defect is a major reason for the poor mechanical properties of concrete in tension and for its brittle character.
On the final scale, macrocracking defects are observed. The opening of these cracks varies from a few hundreds of xcexcm to a few mm. These cracks are coalescent.
Major defects several millimetres in size may also be observed, these being due to poor preparation of the concrete (entrained air, faults in filling).
Solutions have been suggested either for decreasing the presence of these various defects or for reducing their effects on the mechanical properties of the concrete.
In order to improve the mechanical properties of concretes, it has been proposed to replace the sand of the cementitious matrix by other, higher-performance, constituents, but the cost of the concrete rises to an unacceptable level for it to be conceivably used widely in civil engineering because of the economic constraints which burden this field.
It has also been proposed to incorporate high-hardness aggregate into the concrete composition, but the amounts involved in order to achieve the desired performance also increase the manufacturing cost of the concrete excessively, given the high cost of such aggregate.
It has also been proposed to improve, sometimes spectacularly, certain mechanical properties of concrete by incorporating into it a high content of reinforcing fibres, namely, typically, a content of 10 to 15% by volume, but this content not only has a very significant effect on the manufacturing cost of the concrete but also makes its mixing, homogenization and possibly its casting too difficult or too critical to be applicable in civil engineering, especially under the working conditions of a construction site.
Also, it has been possible to partially control the microporosity by decreasing the water-to-cement weight ratio and by using plasticizers. The use of fine fillers, especially pozzolanic-reaction fillers, has also allowed the size of the micropores to be reduced.
However, the organization of the aggregate skeleton by the usual methods does not make it possible to obtain concrete with a satisfactory rheology under acceptable civil engineering operating conditions (poorly dispersed fibres, microstructural defects, etc.).
Microcracking itself has been greatly reduced by:
improving the homogeneity of the concrete, for example by limiting the size of the aggregate to 800 xcexcm;
improving the compactness of the material (aggregate optimization and optional pressing before and during the setting phase);
carrying out heat treatments after setting.
With regard to macrocracking, this may be controlled by the use of metal fibres, but with the operating difficulties mentioned above.
By way of an illustrative document of the prior art, mention may be made of Patent Application WO-A-95/01316 which relates to a metal-fibre-reinforced concrete in which the fibre content is controlled and the fibre dimensions are chosen in defined proportions with respect to those of the aggregate particles.
This fibre-reinforced concrete comprises cement, aggregate particles, fine pozzolanic-reaction particles and metal fibres. The aggregate particles must have a maximum size D of at most 800 xcexcm, the fibres must have an individual length 1 of between 4 and 20 mm and the ratio R of the average length L of the fibres to D must be at least equal to 10, the fibre content being such that the fibres occupy a volume of from 1 to 4% of the volume of the concrete after it has set.
The concrete obtained exhibits ductile behaviour or undergoes pseudo-work-hardening.
There is still a need to remove the aforementioned defects or to greatly reduce their effects, especially microcracks, as it may be seen that the work described in the prior art serves mainly to prevent the development of macrocracks and not of microcracks; microcracks are then only partially stabilized and develop under load.
The object of the present invention is to provide a concrete containing metal reinforcing fibres and having improved properties compared with similar concretes of the prior art.
Improved properties should be understood to mean both mechanical properties that are superior to those of known fibre-reinforced concretes and properties that are at least equal to those of known fibre-reinforced concretes, but these properties being achievable on an industrial scale in a constant and reproducible manner.
Another object of the present invention is to increase the stress level at which the first damage (i.e. microcracks) appears in the concrete and thus to increase the range of use of the concrete, namely the linear elastic behaviour of the concrete.
Yet another object of the present invention is to improve the work hardening of the concrete beyond the first damage by controlling the propagation of macrocracks. The purpose of the invention is thus to increase the range of use of the concrete beyond the first damage by improving the ductile behaviour of the concrete.
Another object of the invention is also to improve, by a synergy effect between the cementitious matrix and the fibres, the behaviour of the concrete both with respect to the appearance of microcracks and to the propagation of macrocracks.
xe2x80x9cCementitious matrixxe2x80x9d should be understood to mean the hardened cementitious composition apart from the metal fibres.
Yet another object of the present invention, which is particularly important for obtaining concrete structures which, because of their size or the work site conditions, could not undergo a heat treatment, is to obtain, under improved conditions over the prior art and especially at temperatures close to ambient temperature (20xc2x0 C.), a concrete having mechanical properties (in the sense mentioned above) at least equal to those which can only be obtained at the cost of a heat treatment in the case of the best known fibre-reinforced concretes.
In addition, the subject of the present invention is the cementitious matrix, which allows the concrete of the invention to be produced, and the premixes which comprise all or some of the constituents necessary for preparing this matrix or the concrete.
In its general form, the invention relates to a concrete consisting of a hardened cementitious matrix in which metal fibres are dispersed, obtained by mixing, with water, a composition which comprises, apart from the fibres:
(a) cement;
(b) aggregate particles having a maximum particle size Dmax of at most 2 mm, preferably at most 1 mm;
(c) pozzolanic-reaction particles having an elementary particle size of at most 1 xcexcm, preferably at most 0.5 xcexcm;
(d) constituents capable of improving the toughness of the matrix, these being chosen from acicular or flaky particles having an average size of at most 1 mm and present in a proportion by volume of between 2.5 and 35% of the combined volume of the aggregate particles (b) and of the pozzolanic-reaction particles (c);
(e) at least one dispersing agent; and satisfying the following conditions:
(1) the percentage by weight of water w with respect to the combined weight of the cement (a) and of the particles (c) is in the range 8-24%;
(2) the fibres have an individual length 1 of at least 2 mm and an l/d ratio of at least 20, d being the diameter of the fibres;
(3) the ratio R of the average length L of the fibres to the maximum particle size Dmax of the aggregate particles is at least 10;
(4) the amount of fibres is such that their volume is less than 4% and preferably less than 3.5% of the volume of the concrete after it has set.
Thus, by virtue of a novel design of the aggregate skeleton and of its relationship with the reinforcing fibres, this approach solves the problem posed with this rheology/mechanical properties compromise.
The properties of the concrete according to the invention are not appreciably changed if aggregate particles (b) having a particle size exceeding 2 mm are also used within the matrix but in a proportion which does not exceed 25% of the volume of the combination of constituents (a)+(b)+(c)+(d).
The presence of this aggregate class in such a proportion may be regarded as a filler which does not contribute to the mechanical performance of the material in so far as:
the D50 particle size of the combination of constituents (a), (b), (c) and (d) is at most 200 xcexcm, preferably at most 150 xcexcm; and
the ratio R of the average length L of the fibres to the D75 particle size of the combination of constituents (a), (b), (c) and (d) is at least 5, preferably at least 10.
D75 particle size and D50 particle size should be understood to mean, respectively, the sizes of the screens whose undersize constitutes 75% and 50%, respectively, of the total volume of the particles.
The invention therefore also relates to a concrete consisting of a hardened cementitious matrix in which metal fibres are dispersed, obtained by mixing, with water, a composition which comprises, apart from the fibres:
(a) cement;
(b) aggregate particles;
(c) pozzolanic-reaction particles having an elementary particle size of at most 1 xcexcm, preferably at most 0.5 xcexcm;
(d) constituents capable of improving the toughness of the matrix, these being chosen from acicular or flaky particles having an average size of at most 1 mm and present in a proportion by volume of between 2.5 and 35% of the combined volume of the aggregate particles (b) and of the pozzolanic-reaction particles (c);
(e) at least one dispersing agent; and satisfying the following conditions:
(1) the percentage by weight of water W with respect to the combined weight of the cement (a) and of the particles (c) is in the range 8-24%;
(2) the fibres have an individual length 1 of at least 2 mm and an l/d ratio of at least 20, d being the diameter of the fibres;
(3) the ratio R of the average length L of the fibres to the D75 particle size of the combination of constituents (a), (b), (c) and (d) is at least 5, preferably at least 10;
(4) the amount of fibres is such that their volume is less than 4% and preferably less than 3.5% of the volume of the concrete after it has set;
(5) the combination of the constituents (a), (b), (c) and (d) has a D75 particle size of at most 2 mm, preferably at most 1 mm, and a D50 particle size of at most 150 xcexcm, preferably at most 100 xcexcm.
Conditions (3) and (5) apply to all the solid constituents (a), (b), (c) and (d) all together, excluding fibres, and not for each constituent taken individually.
Preferably, the toughness of the cementitious matrix is at least 15 J/m2, advantageously at least 20 J/m2.
The toughness is expressed either in terms of stress (stress intensity factor: Kc) or in terms of energy (critical strain energy release rate: Gc), using the formalism of linear fracture mechanics.
The measurement methods used to determine the toughness of the cementitious matrix will be described below in the part of the description pertaining to the examples.
The toughness of the cementitious matrix is obtained by adding to the cementitious composition particles (d) of average size of at most 1 mm, preferably at most 500 xcexcm, these being in acicular form or in the form of flakes. They are present in a proportion by volume lying in the range 2.5-35%, in particular in the range 5-25%, of the combined volume of the aggregate particles (b) and of the pozzolanic-reaction particles (c).
On account of their function to improve the toughness of the matrix, the said particles will be called hereafter in the description xe2x80x9creinforcing particlesxe2x80x9d.
The term xe2x80x9csizexe2x80x9d of the reinforcing particles should be understood to mean the size of their largest dimension (especially the length in the case of those of acicular form).
These may be natural or synthetic products.
The reinforcing particles of acicular form may be chosen from among wollastonite fibres, bauxite fibres, mullite fibres, potassium titanate fibres, silicon carbide fibres, cellulose or cellulose-derivative fibres, such as cellulose acetate, carbon fibres, calcium carbonate fibres, hydroxyapatite fibres and other calcium phosphates, or derived products obtained by grinding the said fibres and mixtures of the said fibres.
Preferably, reinforcing particles are used whose acicularity, expressed by the length/diameter ratio, is at least 3 and preferably at least 5.
Wollastonite fibres have given good results. Thus, the presence of wollastonite fibres in the cementitious matrix leads to a reduction in the microporosity. This surprising effect is particularly apparent in the case of concretes which have undergone 20xc2x0 C. maturing (see below).
The reinforcing particles in the form of flakes may be chosen from among mica flakes, talc flakes, mixed silicate (clay) flakes, vermiculite flakes, alumina flakes and mixed aluminate or silicate flakes and mixtures of the said flakes.
Mica flakes have given good results.
It is possible to use combinations of these various forms or types of reinforcing particles in the composition of the concrete according to the invention.
At least some of these reinforcing particles may have, on their surface, a polymeric organic coating which comprises a latex or is obtained from at least one of the following compounds: polyvinyl alcohol, silanes, siliconates, siloxane resins, polyorganosiloxanes or products from reaction between (1) at least one carboxylic acid containing from 3 to 22 carbon atoms, (2) at least one polyfunctional aliphatic or aromatic amine or substituted amine, containing from 2 to 25 carbon atoms and (3) a crosslinking agent which is a water-soluble metal complex containing at least one metal chosen from among: zinc, aluminium, titanium, copper, chromium, iron, zirconium and lead; this product is more particularly described in Application EP-A-0,372,804.
The thickness of this coating may vary between 0.01 and 10 xcexcm, preferably between 0.1 and 1 xcexcm.
The latices may be chosen from among styrene-butadiene latices, acrylic latices, styrene-acrylic latices, methacrylic latices and carbonylated and phosphonated latices. Latices having functional groups which complex with calcium are preferred.
The polymeric organic coating may be obtained by treating, in a fluidized bed or by using a FORBERG-type mixer, the reinforcing particles in the presence of one of the compounds defined above.
The following compounds are preferred: H240 polyorganosiloxane, Manalox 403/60/WS and WB LS 14 and Rhodorsil 878, 865 and 1830 PX siloxane resins, all sold by Rhodia-Chimie, and potassium siliconates.
This type of treatment is particularly recommended for reinforcing particles which are naturally occurring substances.
With regard to the metal fibres, these may be metal fibres chosen from among steel fibres, such as high-strength steel fibres, amorphous steel fibres or stainless steel fibres. Optionally, the steel fibres may be coated with a non-ferrous metal such as copper, zinc, nickel (or their alloys).
The individual length 1 of the metal fibres is at least 2 mm and is preferably in the 10-30 mm range. The l/d ratio is at least 20, and preferably at most 200, d being the diameter of the fibres.
Fibres having a variable geometry may be used: they may be crimped, corrugated or hooked at the ends. The roughness of the fibres may also be varied and/or fibres of variable cross-section may be used; the fibres may be obtained by any suitable technique, including by braiding or cabling several metal wires, forming a twisted assembly.
The fibre content is such that the fibres occupy a volume of less than 4%, and preferably of less than 3.5%, of the volume of the concrete after it has set.
Advantageously, the average bonding stress of the fibres in the hardened cementitious matrix must be at least 10 MPa, preferably at least 15 MPa. This stress is determined by a test comprising the extraction of a single fibre embedded in a block of concrete, as will be described below.
It has been observed that the concretes according to the invention, having both such a fibre-bonding stress and a high matrix toughness, (preferably of at least 15 J/m2), result in superior mechanical performance, by synergy between these two properties.
The level of fibre/matrix bonding may be controlled by several means, which may be used individually or simultaneously.
According to a first means, the bonding of the fibres in the cementitious matrix may be achieved by treating the surface of the fibres. This fibre treatment may be carried out by at least one of the following processes:
fibre etching;
deposition of a mineral compound on the fibres, especially by depositing silica or a metal phosphate.
The etching may be carried out, for example, by bringing the fibres into contact with an acid, followed by neutralization.
Silica may be deposited by bringing the fibres into contact with silicon compounds, such as silanes, siliconates or silica sols.
In general, a metal phosphate is deposited using a phosphatizing process, which consists in introducing prepickled metal fibres into an aqueous solution comprising a metal phosphate, preferably manganese phosphate or zinc phosphate, and then in filtering the solution in order to recover the fibres. Next, the fibres are rinsed, neutralized and then rinsed again. Unlike the usual phosphatizing process, the fibres obtained do not have to undergo grease-type finishing; however, they may be optionally impregnated with an additive either in order to provide anticorrosion protection or to make it easier for them to be processed with the cementitious medium. The phosphatizing treatment may also be carried out by coating or spraying the metal phosphate solution onto the fibres.
Any type of phosphatizing process may be usedxe2x80x94reference may be made on this subject to the treatments described in the article by G. LORIN entitled xe2x80x9cThe Phosphatizing of Metalsxe2x80x9d (1973), Pub. Eyrolles.
According to a second means, the bonding stress of the fibres in the cementitious matrix may be achieved by introducing into the composition at least one of the following compounds: silica compounds comprising mostly silica, precipitated calcium carbonate, polyvinyl alcohol in aqueous solution, a latex or a mixture of the said compounds.
The phrase xe2x80x9csilica compound comprising mostly silicaxe2x80x9d should be understood here to mean synthetic products chosen from among precipitated silicas, silica sols, pyrogenic silicas (of the Aerosil type), aluminosilicates, for example Tixosil 28 sold by Rhodia Chimie, or clay-type products (either natural or derived), for example smectites, magnesium silicates, sepiolites and montmorillonites.
It is preferred to use at least one precipitated silica.
Precipitated silica should be understood here to mean a silica obtained by precipitation from the reaction of an alkali metal silicate with an acid, generally an inorganic acid, with a suitable pH of the precipitation medium, in particular a basic, neutral or slightly acid pH; any method may be used to prepare the silica (addition of acid to a silicate sediment, total or partial simultaneous addition of acid or of silicate to a water or silicate-solution sediment, etc.), the method being chosen depending on the type of silica that it is desired to obtain; after the precipitation step there generally follows a step of separating the silica from the reaction mixture using any known means, for example a filter press or a vacuum filter; a filter cake is thus collected, which is washed if necessary; this cake may, optionally after crumbling, be dried by any known means, especially by spray drying, and then optionally ground and/or agglomerated.
In general, the amount of precipitated silica introduced is between 0.1% and 5% by weight, expressed as dry matter, with respect to the total weight of the concrete. Above 5%, rheologie problems during preparation of the mortar usually arise.
Preferably, the precipitated silica is introduced into the composition in the form of an aqueous suspension. This may especially be an aqueous silica suspension having:
a solids content of between 10 and 40% by weight;
a viscosity of less than 4xc3x9710xe2x88x922 Pa.s for a shear of 50 sxe2x88x921;
an amount of silica contained in the supernatant liquid of the said suspension after centrifuging at 7500 rpm for 30 minutes of more than 50% of the weight of the silica contained in the suspension.
This suspension is more particularly described in Patent Application WO-A-96/01787. The Rhoximat CS 60 SL silica suspension sold by Rhodia Chimie is particularly suitable for this type of concrete.
The cement (a) of the composition according to the invention is advantageously a Portland cement, such as the Portland cements CPA PMES, HP, HPR, CEM I PMES, 52.5 or 52.5R or HTS (high silica content).
The aggregate particles (b) are essentially screened or ground sands or mixtures of sands, which advantageously may comprise silicious sands, particularly quartz flour.
The maximum particle size D100 or Dmax of these particles is preferably at most 6 mm.
These aggregate particles are generally present in an amount of 20 to 60% by weight of the cementitious matrix, preferably from 25 to 50% by weight of the said matrix.
The fine pozzolanic-reaction particles (c) have an elementary particle size of at least 0.1 xcexcm and at most 1 xcexcm, preferably at most 0.5 xcexcm. They may be chosen from among silica compounds, especially silica fume, fly ash, blast-furnace slag and clay derivatives, such as kaolin. The silica may be a silica fume coming from the zirconium industry rather than a silica fume coming from the silicon industry.
The water-cement weight ratio, conventional in concrete technology, may vary when cement substitutes, especially pozzolanic-reaction particles, are used. For the needs of the present invention, the weight ratio of the amount of water W with respect to the combined weight of the cement to the pozzolanic-reaction particles has therefore been defined. This ratio, thus defined, is between approximately 8 and 24%, preferably between approximately 13 and 20%. However, in the description of the examples, the water-to-cement ratio W/C will be used.
The composition according to the invention also comprises at least one dispersing agent (e). This dispersing agent is generally a plasticizer. The plasticizer may be chosen from among: lignosulphonates, casein, polynaphthalenes, particularly polynaphthalene-sulphonates of alkali metals, formaldehyde derivatives, polyacrylates of alkali metals, polycarboxylates of alkali metals and grafted polyethylene oxides. In general, the composition according to the invention comprises from 0.5 to 2.5 parts by weight of plasticizer per 100 parts by weight of cement.
Other additives may be added to the composition according to the invention, for example an anti-foam agent. By way of example, anti-foam agents based on polydimethylsiloxane or on propylene glycol may be used.
Among agents of this type, mention may be made especially of silicones in the form of a solution or in the form of a solid or, preferably, in the form of a resin, an oil or an emulsion, preferably in water. Most particularly suitable are silicones essentially comprising M repeat units (RsiO0.5) and D repeat units (R2SiO). In these formulae, the radicals R, which may be identical or different, are more particularly chosen from among hydrogen and alkyl radicals comprising 1 to 8 carbon atoms, the methyl radical being preferred. The number of repeat units is preferably in the 30 to 120 range.
The amount of such an agent in the composition is generally at most 5 parts by weight per 100 parts of cement.
All the sizes of the particles are measured by TEM (transmission electron microscopy) or SEM (scanning electron microscopy).
The matrix may also contain other ingredients as long as they do not prejudice the expected performance of the concrete.
The concrete may be obtained according to any process known to those skilled in the art, especially by mixing the solid constituents with water, forming (moulding, casting, injection, pumping, extrusion, calendering) and then hardening.
For example, in order to prepare the concrete the constituents of the matrix and the reinforcing fibres are mixed with the suitable amount of water.
Advantageously, the following order of mixing is respected:
mixing of the pulverulent constituents of the matrix (for example for 2 minutes);
introduction of the water and a fraction, for example half, of the admixtures;
mixing (for example for 1 minute);
introduction of the remaining fraction of the admixtures;
mixing (for example for 3 minutes);
introduction of the reinforcing fibres and the additional constituents;
mixing (for example for 2 minutes).
The concrete then undergoes maturing between 20xc2x0 C. and 100xc2x0 C. for the time necessary to obtain the desired mechanical properties.
Surprisingly, it has been found that maturing at a temperature close to ambient temperature gave good results, this being so by virtue of the choice of constituents in the composition of the concrete.
In this case, the concrete is left to mature at, for example, a temperature close to 20xc2x0 C.
The maturing process may also involve a heat treatment between 60 and 100xc2x0 C. at normal pressure on the hardened concrete.
The concrete obtained may be especially subjected to a heat treatment between 60 and 100xc2x0 C. for between 6 hours and 4 days, with the optimum time being about 2 days and the treatment starting after the end of the mixture setting phase or at least one day after the onset of setting. In general, treatment times of 6 to 72 hours are sufficient within the aforementioned temperature range.
The heat treatment is carried out in a dry or wet environment or carried out according to cycles alternating between the two environments, for example 24 hours in a wet environment followed by 24 hours in a dry environment.
This heat treatment is implemented on concretes which have completed their setting phase, these preferably being aged for at least one day and better still aged for at least approximately 7 days.
The addition of quartz powder may be useful when the concrete is subjected to the aforementioned heat treatment.
The concrete may be pretensioned, by bonded wires or by bonded tendons, or post-tensioned, by single unbonded tendons or by cables or by sheaths bars, the cable consisting of an assembly of wires or consisting of tendons.
The prestressing, whether in the form of pretensioning or in the form of post-tensioning, is particularly well suited to products made of the concrete according to the invention.
This is because metal prestressing cables always have a very high, ill-used, tensile strength since the brittleness of the matrix which contains them does not allow the dimensions of the concrete structural elements to be optimized.
Progress has already been made in terms of the use of high-performance concretes; in the case of the concrete according to the invention, the material is homogeneously reinforced by metal fibres allowing it to achieve high mechanical performance in conjunction with ductility. The prestressing of this material, by means of cables or tendons, whatever the pretensioning mode, is then used almost to its full amount, thereby creating prestressed concrete elements that are very strong both in tension and in bending, and are therefore optimized.
The reduction in volume obtained, because of this increase in mechanical strength, can produce very light prefabricated elements. Consequently, there is then the possibility of having long-span concrete elements that are easily transportable because of their lightness; this is particularly well suited to the construction of large structures in which the use of post-tensioning is widely employed. In the case of this type of structure, the solution provides particularly favourable savings to be made in terms of work-site duration times and assembly.
In addition, in the case of a thermal cure, the use of pretensioning or post-tensioning significantly reduces shrinkage.
This property is particularly desirable and all of the above advantages associated with the very low permeability of the productxe2x80x94highly advantageous in the case of durability and maintenance of structures over timexe2x80x94mean that this material may validly be substituted for structures made of steel.
The concretes obtained according to the present invention generally have a direct tensile strength Rt of at least 12 MPa.
They may also have a flexural strength Rf in 4-point bending of at least 25 MPa, a compressive strength Rc of at least 150 MPa and a fracture energy Wf of at least 2500 J/m2.
The invention also relates to the cementitious matrix intended for obtaining and for employing the concrete defined above.
Finally, the invention relates to premixes comprising all or some of the constituents necessary to prepare the concrete and the matrix defined above.